It is known as disclosed in U.S. Pat. No. Re 28,223 to Odenthal et al to use a dome-shaped mesh electrode in a cathode ray tube to expand the scan of an electron beam thereof. It is not desirable to use a dome-shaped mesh electrode because it becomes contaminated and because the mesh intercepts the beam thereby minimizing the number of electrons that reach the fluorescent screen, and the mesh creates a multiplicity of lenses which introduce aberrations into a well-focussed electron beam.
U.S. Pat. No. 3,496,406 to J. Deschamps is directed to a cathode ray tube having an electrostatic quadrupole lens downstream from horizontal deflection plates which is disposed within a dome-shaped electrode having a slot therethrough. The combination of the quadrupole lens and dome-shaped electrode constitutes a lens system which causes the paths of electrons to cross over in the vertical plane and to be accelerated through the slot in the dome-shaped electrode so that the focussed electron beam impinges on the fluorescent screen.
U.S. Pat. No. 3,792,303 to Albertin et al is an improvement of the Deschamps cathode ray tube in that the Albertin et al invention uses correcting electrodes disposed at either side of the quadrupole lens in order to correct for pin cushion distortion, i.e., the bowing of the horizontal and vertical lines. These quadrupole lens and dome-shaped electrode structures are difficult to manufacture and to position relative to each other when being mounted within a cathode ray tube thereby introducing aberrations into the electron beam which results in less brightness of the image being displayed on the fluorescent screen.
Klemperer in U.S. Pat. No. 2,412,687 teaches the basic concept of an electron lens for use in a cathode ray tube having aligned tubular members which are provided with interdigitated sections defining nonrotationally-symmetrical lens with two-fold symmetry, but the tubular members do not have interdigitated sections which are provided with parts having different radii to provide distortion-free imaging.